We identified microRNA expression patterns associated with diagnosis, prognosis, and therapeutic outcome in colon adenocarcinoma. MircoRNAs associated with diagnosis and prognosis were first identified in a cohort recruited from the NCI-Maryland Colon Cancer Case-Control Study. These findings were validated in an independent cohort recruited from Hong Kong. This study was the largest to date profiling microRNA expression patterns in colon tumors and the only study to analyze associations with prognosis or therapeutic outcome. Thirty-seven microRNAs were differentially expressed in tumors. Five microRNAs were selected for validation by quantitative reverse transcriptase polymerase chain reaction (RT-PCR). They were miR-20a, miR-21, miR-106a, miR-181b, and miR-203 and all were increased in tumors in the validation cohort. Higher miR-21 expression was also found in colon adenomas indicating altered expression of this mircoRNA may be an early event in colon carcinogenesis. We also found that tumors with high expression of miR-21 are associated with poor survival outcome and poor therapeutic outcome to adjuvant chemotherapy in both cohorts, independent of staging and other clinical covariates. Therefore, miR-21 expression is a strong candidate as a biomarker of diagnosis, prognosis, and therapeutic outcome of colon adenocarcinomas.